1. Field of the Invention
This invention relates to a printing apparatus which receives a video signal as input data and prints and records an image, and more particularly to a sampling circuit of the video signal.
2. Description of the Prior Art
As accuracy, density and resolution of image display devices in the recent CAD CAM applications have made a progress, it has now becomc necessary for hard copy apparatuses, too, to reproduce the displayed image with a high level of fidelity. In order to particularly effect high fidelity recording of graphic display, sampling of an image signal by a sampling clock on the 1:1 basis must be made.
Some display devices having low resolution supply a sync clock signal corresponding to a pixel signal together with a display video signal, and sampling of an image signal by this clock signal has been made in the past.
When display devices do not have such a sync clock signal, there has been employed conventionally a clock signal generator which has some multiples of frequency of the sync clock signal described above and a frequency division circuit and synchronizes the phase of the frequency division circuit by a video sync signal.
In latest high resolution display devices described above, however, the video signal frequency is as high as above 100 MHz and hardly any of them supply the sync clock signal. In a system which uses a frequency division circuit, too, there is a limit to circuit elements which operate at several hundreds of mega-Hertz and such a circuit cannot be accomplished easily. In such a high speed video signal, variance of the delay quantities of the circuit elements used frequently causes variance of phase between the video sync signal and the image signal. In color display devices, phases mostly vary among R, G and B signals.